1. Field of the Invention
This invention relates to an improvement in a control system for an automatic transmission for a vehicle.
2. Description of the Prior Art
In a conventional automatic transmission of a vehicle, which is coupled to a hydro-dynamic power transmitting device such as a torque converter or a fluid coupling, and designed to attain a plurality of gear ratios by selectively engaging a plurality of frictional engaging elements, a gear shift can be obtained by switching engagement from one of the frictional engaging elements to another.
With an automatic transmission of this kind, however, there is a moment when the output-shaft torque, as indicated by a dashed line in FIG. 6(a), of the transmission drops (indicated by reference character A), because of the structural feature thereof, during an initial stage of a gear shift. The drop in the output-shaft torque is particularly enlarged in the case of a shift between the first speed and the second speed whose gear ratios differ widely, resulting in a shift shock impairing the smoothness of the shift.
When upshifting from the first speed to the second speed, the torque ratio drops down to the level of the second speed while the transmission ratio is still remaining at the level of the first speed at point A in FIG. 6(a), as a consequence of which the output-shaft torque temporarily drops by the amount equal to the difference, between the gear ratio of the first speed and the gear ratio of the second speed, multiplied by the torque input to the input shaft of the transmission. Then, the output-shaft torque is increased again by the engaging torque of the frictional engaging elements that engage in the second speed and by an inertial force caused by acceleration and deceleration of each rotary element, and the speed ratio becomes that of the second speed when said acceleration and deceleration of each rotary element ends, thus completing the shift. As the inertial force of the rotary elements disappears, the output-shaft torque drops to the level of the second speed.
Referring now to FIGS. 9 to 17, the changes in the speed ratio and torque ratio involved in the aforementioned gear shift will be discussed in detail.
FIG. 11 is a speed diagram showing the relationship among the rotational speeds of a forward sun gear (F/S), an annulus gear (A/G), a carrier (C) and a reverse sun gear (R/S) which are the four rotary elements in a ravigneaux type planetary gear unit providing three forward speeds as shown in FIGS. 9 and 10.
In this speed diagram, the length corresponding to the reciprocal ratio of the radius of each rotary element with respect to the carrier (C) is shown along the axis of abscissa, each length being divided into portions shown on both sides of the carrier (C) according to the directions in which the rotary element turns when the carrier (C) is fixed. Along the axis of ordinate is indicated the speed of rotation of each rotary element. The relationship among the rotational speeds of the individual rotary elements is shown by a single straight line (such as a straight line AA indicating the first speed condition). Assuming that the inertial force of each rotary member is a mass, the torque exerted thereby is a force, and each of the straight lines (AA, BB and CC) in FIG. 11 is a lever, the dynamic characteristics of force working on each rotary element can be explained.
For example, a process to immobilize the reverse sun gear (R/S) by actuating a kickdown brake (KDB) when upshifting from the first speed to the second speed corresponds to a change from the straight line AA to the straight line BB, with a resulting change in the force acting on the annulus gear (A/G) which corresponds to a change in the output-shaft torque.
In FIG. 11, reference characters Z.sub.1, Z.sub.2 and Z.sub.4 indicate the number of teeth of the forward sun gear (F/S), annulus gear (A/G) and reverse sun gear (R/S), respectively.
Using corresponding speed diagrams, changes in torque occurring in time spans T1 to T4 during the upshifting from the first speed to the second speed will be described in the following.
First, it is to be assumed that the input torque (or the amount of opening of the throttle valve of an engine that typically represents the input torque) and the vehicle speed remain constant during the upshift sequence.
FIG. 14 shows time span T.sub.1 (ranging from time 0 to time t.sub.1) in which the first speed is still maintained. This state may correspond to a lever carrying weights W.sub.1 to W.sub.4, which respectively correspond to the inertia of the forward sun gear (F/S), body, carrier (C) and reverse sun gear (R/S), that is pushed up at one end by input torque T.sub.1 about a one-way clutch (OWC) that serves as a fulcrum. If the gear ratio of the first speed is 1:2.846, the weight (body) W.sub.2 is pushed up by a force equal to 2.846T.sub.1 when input torque T.sub.1 is given, as a consequence of which a downward force (torque) equal to 2.846T.sub.1 works on the lever. This force (torque) exerted on the lever is output-shaft torque T.sub.0.
Next, in FIG. 15, time span T.sub.2 (ranging from time t.sub.1 to time t.sub.2) is shown in which shifting from the first to the second speed takes place by operating the kickdown brake (KDB) to stop the rotation of the reverse sun gear (R/S). At this time, torque works in the direction to raise the reverse sun gear (R/S), the mass of which being W.sub.4, in FIG. 15. As the engaging force T.sub.4 of the kickdown brake (KDB) increases gradually to increase the force to push up the mass W.sub.4, the force of the one-way clutch (OWC) to support the lever gradually decreases and eventually becomes zero. This change is accompanied by a shift of fulcrum from the one-way clutch (OWC) to the reverse sun gear (R/S). Also, the relationship between the input and the output forces (torques) changes from one for the first speed to that of the second speed. If the gear ratio of the second speed is 1:1.581, output torque T.sub.0 decreases from 2.846T.sub.1 to 1.581T.sub.1.
It should be noted that when the force working on the one-way clutch (OWC) becomes zero or T.sub.4 becomes equal to 0.581T.sub.1, the form of the speed diagram remains unchanged, which means that the speed ratio between the input and the output shafts remains the same as that for the first speed (2.846:1). However, the torque ratio between the two shafts has now changed to that of the second speed (1.581:1). For this reason, a temporary drop in the output-shaft torque, such as the one that is seen in time span T.sub.2, occurs, which is unavoidable because of the structural characteristics of a multispeed transmission with gear train.
FIG. 16 shows time span T.sub.3 (ranging from time t.sub.2 to time t.sub.3) in which the speed ratio has not changed to that of the second speed although the torque ratio is already that of the second speed. To attain the speed ratio of the second speed, the engaging torque T.sub.4 of the kickdown brake (KDB) is increased further so that the reverse sun gear (R/S), the mass of which being W.sub.4, is pushed up. Since the vehicle speed is assumed to be constant, the lever turns about the body whose mass is W.sub.2, thereby raising each of the inertial masses mentioned before. Then, the resulting reaction force works on point W.sub.2 of the lever as an inertial torque .alpha..
Therefore, the force (torque) working on the lever becomes 1.581T.sub.1 +.alpha., resulting in an increase in the output-shaft torque.
It takes a certain amount of time before the second speed is achieved after the output torque began to increase. It is due to the time needed to increase or decrease the rotational inertia of each of the rotary elements. The length of the time is dependent on the length of T.sub.1 and T.sub.4. When the value of T.sub.4 is larger, shifting time becomes shorter but shift shock increases.
FIG. 17 shows time span T.sub.4 (time t.sub.3 and onward) in which the rotation of the reverse sun gear (R/S) is stopped by the kickdown brake (KDB), the speed ratio becomes that of the second speed, and the inertial force .alpha. resulting from the motion of the inertial masses reduces to zero. Consequently, output torque T.sub.0 decreases to 1.581T.sub.1 which is appropriate to the second speed.
As the inertial torque reduces, there is no more excessive slip in the torque converter, and the amount of slip returns to its normal level at which enough torque for the second speed can be transmitted.